Which sequence correctly lists the four steps of the cross-bridge cycle in order?

• A. Cross-bridge formation; Power stroke; Cross-bridge detachment; Cocking of the myosin head
• B. Power stroke; Cross-bridge formation; Cocking; Detachment
• C. Cocking; Cross-bridge formation; Power stroke; Detachment
• D. Cross-bridge detachment; Cocking; Cross-bridge formation; Power stroke

Answer: (A)

In the cross-bridge cycle, the myosin head must be energized before it can bind to actin and pull. The correct order starts with ATP being bound to myosin and hydrolyzed to ADP and Pi, which “cock’s” the head into a high-energy orientation. With the head cocked, it can form a cross-bridge by binding to exposed sites on actin. Release of Pi then triggers the power stroke, where the myosin head pivots and pulls the actin filament toward the center of the sarcomere. After the power stroke, ADP is released, and a new ATP binds to myosin, causing detachment from actin and readying the cycle to repeat. Ca2+ helps this process by moving tropomyosin away from actin’s binding sites, enabling cross-bridge formation once the head is cocked. If a sequence lists cross-bridge formation before cocking, it skips the necessary energized state and doesn’t fit how the cycle actually proceeds.